TBS: Document discard_on_write_failure + expose it to the APM Integration

[lucabelluccini]

Potential follow up of #15159

• Docs: document discard_on_write_failure
• Integration (APM): expose the setting via
  • https://github.com/elastic/integrations/blob/a55e9eecb844d1d9192eb8584b81c687a213ef1b/packages/apm/agent/input/template.yml.hbs#L74
  • https://github.com/elastic/integrations/blob/main/packages/apm/manifest.yml

[carsonip]

It was intentional to have sampling.tail.discard_on_write_failure undocumented, as it was supposed to be an escape hatch for users who are facing TBS storage limit issues while we look for a long term solution.

I understand the pain to configure this config (and many more configs not exposed in apm integration package) and I agree we should make it easier.

Regarding documentation, I'm not sure if the team is committed to support this config in the long run. cc @simitt